I. Field of the Invention
This invention relates to Fabry-Perot modulators and in particular, but not exclusively, to those employing a multiple quantum well modulator.
II. Related Art and Other Considerations
Since the first observation of the effect of electric fields on the optical properties of GaAs-AlGaAs multiple quantum well (MQW) structures a number of opto-electronic devices have been demonstrated which exploit their enhanced electro-absorptive properties, e.g. fast intensity modulators and hybrid optical logic elements, both bistable and non-bistable. These devices employ an MQW layer grown epitaxially as the intrinsic region of a pin diode that can operate as an electro-absorptive modulator and efficient photodetector simultaneously.
A contrast ratio (on:off) of .apprxeq.2:1 has been observed in devices with only .apprxeq.1 .mu.m of MQW absorber usually consisting of well and barriers .apprxeq.100 .ANG. thick. This is very efficient, given the device size, but a better contrast ratio is desirable. By `contrast ratio` is meant the ratio of the high: low output states, irrespective of whether the device switches on or off with applied bias. `Modulation` is the absolute change in state, which in reflection or transmission terms can only be between 0 and 1.
It would at first seem obvious simply to increase the thickness of the MQW layer to obtain better modulation figures. However, the situation is complicated by the background doping level of this layer which up to now has had a lower limit of .apprxeq.1.times.10.sup.15 cm.sup.3 in the best available material, and is routinely two or three times this value. This causes a significant fall-off in the electric field across the intrinsic region of the pin device which broadens the absorption edge of the MQW material even at zero bias and, moreover, produces a different red shift of the excitonic absorption in each well as an external bias is applied to the device for modulation. As the bias is increased the absorption edge broadening becomes worse due to the roughly parabolic dependence of the edge shift on applied field. So, instead of producing a larger change in the intensity of a transmitted or reflected beam of light the increase in thickness of the absorbing layer might only serve to distribute absorption changes over a wider spectral region and leave the modulation at the operating wavelength relatively unaffected.
Optimisation calculations have shown that if the residual doping is 2.times.10.sup.15 /cm.sup.3 it is best to use about 45 wells of 100 .ANG. GaAs separated by barriers of 100 .ANG. Al.sub.0.3 Ga.sub.0.7 As confirming the limit of around 1 .mu.m for the total thickness.
A second problem with increasing the MQW thickness is the additional bias voltage required to induce a given change in absorption. This would also be the main disadvantage in using narrower quantum wells e.g. 60 .ANG., where changes in absorption can be much greater because of the increased zero-bias exciton oscillator strength at the absorption edge and the greater resistance of the exciton to field-induced broadening, but the shift of the absorption edge with applied field is much reduced. It has been proposed that MQW devices may be integrated in 2-dimensional arrays with Si-based LSI electronics to form high-bandwidth optical interconnects, and in this case drive voltages for such modulators or logic gates will be limited to a few volts.
One method of improving modulation in a device that has limitations on its absorber thickness and drive voltage is to increase the effective optical path length by incorporating the MQW pin diode into a Fabry-Perot etalon.
An asymmetric Fabry-Perot modulator has been proposed which operates as a reflection modulator with a low reflectivity front surface and a high reflectivity back surface (Whitehead M., Parry G. and Wheatley P. "Investigation of etalon effects in GaAs-AlGaAs multiple quantum well modulators". IEE PROCEEDINGS, Vol. 136, Pt.J,Nol, February 1989 pp 52-58).
Modelling of one such a device demonstrated a maximum change in reflectivity of about 80.degree./o. However, it is desirable to have not only a large absolute change but to have a large modulation depth, or contrast ratio, i.e. the ratio of the maximum and minimum reflectivities. It is an object of the present invention to provide an asymmetric Fabry-Perot modulator having an improved contrast ratio. Accordingly there is provided a Fabry-Perot modulator comprising a resonant cavity defined by front and back reflective surfaces of different reflectivities and including an electro-absorptive means, the absorption of which can be increased by the application of an electrical bias signal to a value such that the reflectivity of the cavity is substantially zero at a resonant frequency of the cavity.